We propose to study the role of plasma membrane-bound platelet enzymes in maintaining platelet function. We have identified a membrane-bound pyridine nucleotide-dependent reductase which is inhibited by quinacrine, an inhibitor of ADP and collagen-induced platelet aggregation. We shall investigate the kinetic and regulatory properties of this enzyme so as to discover modifiers and thus be able to alter the activity of the enzyme. The effect of this altered enzyme activity will be tested by measuring ADP-induced aggregation and serotonin uptake.